My invention comprises improvements in a power steering gear of the kind disclosed in U.S. Pat. Nos. 4,434,866, 4,586,582, 4,570,735, 4,561,521, and 4,570,736. Each of these patents is assigned to the assignee of my present invention. Each of the prior art constructions disclosed in these patents comprises a power steering system having a fluid motor with a double-acting piston which cooperates with a fluid cylinder of the motor to define two opposed working chambers. A power steering pump, driven by the vehicle engine, includes a pressure control valve and a flow control valve so that the flow delivered by the pump to the steering system remains constant regardless of varying engine speed.
The fluid circuit, of which the motor and the pump are a part, includes a rotary valve assembly having an inner valve member connected mechanically to a driver-controlled steering shaft and a rotary valve sleeve connected to the dirigible wheels of the vehicle. The valve assembly comprises registering valve lands that control pressure distribution from the pump to the motor whereby the angular disposition of the dirigible wheels of the vehicle is controlled with a steering effort that is assisted by the steering pressure forces developed by the motor.
The inner and outer valve elements define registering valve lands which cooperate, one with respect to the other, to define a variable area flow path between the outlet side of the pump and the flow return side. Provision is made for connecting each of the opposed working chambers of the fluid motor to the rotary valve assembly so that the pressure developed in the pressure passages connecting the pump with the motor may be distributed to each of the working chambers selectively. When one chamber is pressurized and the other exhausted, the vehicle is adapted to turn in one direction. Upon a reversal in the direction of the pressure differential in the fluid motor, the steering direction reverses.
Since the flow delivered by the pump is constant, the pressure differential developed in one working chamber is a function of the reciprocal of the square of the effective flow area across the valve lands. Thus, the relationship between pressure and valve flow area can be expressed by the equation: ##EQU1## where C and K are constants and A is the effective flow area. Since the flow Q is also a constant, the pressure is functionally related only to the reciprocal of a single variable; namely, the effective flow area. Thus, ##EQU2## where K' is a different characteristic constant that takes into account the value of flow.
Torque is transferred from the steering shaft to the driving pinion of the steering gear through a torsion bar. Upon deflection of the bar, relative angular motion of the elements of the steering valve occurs. Thus, the steering torque is determined by the torsion bar deflection as is the steering assist provided by the fluid motor.
The relationship between torque applied to the torque input shaft and the steering pressure that is developed can be changed by a vehicle speed-sensitive valve in a parallel flow circuit from the pump through the valve system so that at low speeds the pressure is highly sensitive to changes in torque, whereas at high speeds the pressure changes are relatively insensitive to torque. These distinct functional relationships are identified in U.S. Pat. No. 4,561,521 as a low-speed operating mode and as a high-speed operating mode, respectively.
Other prior art references that describe a variable steering assist depending upon speed and employ pressure reaction pistons that resist relative angular displacement of the steering valve elements are French Patent 2,537,936 (1983) and U.S. Pat. No. 4,905,784, dated Mar. 6, 1990.